The conventional intrusion detection method uses cameras to capture images, and determines if an intruder is present by comparing the captured images. By using a camera to capture image, the surveillance area must be well-lit, especially at night in order to guarantee the quality of the captured image. The energy consumption is usually high using the conventional methods. In addition, because the lighting changes in the environment are usually frequent, complicated and unpredictable, false alarms are quite common when using image comparison. For example, a sunny day and an overcast day may generate quite different images; turning off the light in the corridor may affect the lighting condition in a room; the reflection of the surrounding objects in the monitor, and so on. All these problems pose difficult challenges for around-the-clock surveillance using image comparison.
Another technique commonly used in many intrusion detection methods is infrared. The use of passive infrared is to take advantage of the temperature difference among a number of regions, while the use of active infrared relies on the changes of the strength of the reflected signal. The former has the disadvantage of making mistake when surrounding temperature is high or unstable, for example: working heater and flame of the candle. Moreover, it is unable to detect a person covered with an insulating coating. The disadvantage of the active infrared method is that it can only detect a small area, usually along the line of sight. While radar is also used in Doppler methods to detect the speed of objects, it is not an appropriate solution for the indoor as it is too expensive and with a large volume.
The preprocess, feature extraction, and background module training have been used in many audio or image recognition applications and applicable to ultrasonic signals. As the ultrasonic detector can detect different signal features depending on the presence or the absence of an object within the detection range, it does not reply on the lighting and temperature changes in the surroundings. It is possible to exploit this trait in intrusion detection to replace the aforementioned methods troubled by some problems.
U.S. Pat. No. 4,319,349 disclosed an ultrasonic alarm system to detect the Doppler-shifted components which indicate the presence of moving objects in a protected area. It preprocess the transmitter signal and allows much simpler receiver circuitry to detect Doppler-shifted echoes indicating the presence of an intruder while rejecting similar echoes caused by non-intrusive objects which would otherwise produce a false alarm. However, the system has the disadvantage of being unable to detect a static or slowly moving object.
The prior arts mentioned above may be used as automatically controlling systems. However, they are not suitable to be used for intrusion detection, because of being understood their weakness and people can be trained to fool this system.